Wavelength division multiplexed (WDM) optical communication systems are known in which multiple optical signals, each having a different wavelengths, are combined onto a single optical fiber. Such systems typically include a laser associated with each wavelength, a modulator configured to modulate the output of the laser, and an optical combiner to combine each of the modulated outputs.
Conventionally, WDM systems have been constructed from discrete components. For example, the lasers, modulators and combiners have be packaged separately and provided on a printed circuit board. More recently, however, many WDM components have been integrated onto a single chip, also referred to a photonic integrated circuit (PIC).
In order to further increase the data rates associated with WDM systems, various modulation formats have been proposed for generating the modulated output.
One such modulation format, known as polarization multiplexed differential quadrature phase-shift keying (“Pol Mux DQPSK”), can provide higher data rates than other modulation formats. A PolMuxDOPSK signal typically includes light having first and second polarizations, such as transverse TE and transverse TM polarizations, which are modulated independently of each other. In a WDM system generating such signals, four modulators are often provided for each wavelength, such that a first modulator pair modulates light having the first polarization to have respective in-phase and quadrature components, and a second modulator pair modulates light having the second polarization with such components.
The electrical signals used to drive the modulators typically constitute a series of pulses having either a non-return-to-zero (NRZ) or (RZ) return-to-zero pulse shape. RZ signals return to a zero or low level between pulses, whereas NRZ signals do not transition to the zero level between pulses. When supplied to a modulator, the RZ modulator drive signals (or electrical RZ (ERZ) signals) generate corresponding RZ optical signals that may have improved performance compared to NRZ optical signals. Accordingly, RZ optical signals may be preferred in many applications.
As the number of modulators increases on the PIC, however, the complexity and power consumption of the ERZ signal generating circuitry also increases. Accordingly, there is a need for such circuitry that has a relatively simple design and relatively low power consumption. In addition, such circuitry should preferably have controlled timing in order to reduce errors.